Piezoelectric devices such as thin film bulk acoustic resonators (TFBAR) and similar technologies like quartz crystal microbalances (QCM) have been employed as mass detectors for some time. One application of piezoelectric resonators is in detecting very small quantities of materials. Piezoelectric resonators used as sensors in such applications are sometimes called “micro-balances.” A piezoelectric resonator is typically constructed as a thin, planar layer of crystalline or polycrystalline piezoelectric material sandwiched between two electrode layers. When used as a sensor, the resonator is exposed to the material being detected to allow the material to bind on a surface of the resonator.
One conventional way of detecting the amount of the material bound on the surface of a resonator is to operate the resonator as an oscillator at its resonant frequency. As the material being detected binds on the resonator surface, the oscillation frequency of the resonator is reduced. This change in the oscillation frequency of the resonator, presumably caused by the binding of the material on the resonator surface, is measured and used to calculate the amount of the material bound on the resonator or the rate at which the material accumulates on the resonator's surface.
The sensitivity of a piezoelectric resonator in air as a material sensor is theoretically proportional to the square of the resonance frequency. See, e.g., G. Sauerbrey, Zeitschrift für Physik 155 (2): 206-222. Thus, the sensitivities of material sensors based on the popular quartz crystal resonators are limited by their relatively low oscillating frequencies, which typically range from several MHz to about 100 MHz. The development of thin-film resonator (TFR) technology can potentially produce sensors with significantly improved sensitivities. A thin-film resonator is formed by depositing a thin film of piezoelectric material, such as AlN or ZnO, on a substrate. Due to the small thickness of the piezoelectric layer in a thin-film resonator, which is on the order of several microns, the resonant frequency of the thin-film resonator is on the order of 1 GHz. The high resonant frequencies and the corresponding high sensitivities make thin-film resonators useful for material sensing applications.